Although every cancer is unique, similarities do exist with regard to the genetic epidemiology of many of the more common cancers such as breast, ovary, colon and prostate. For each of these cancers, a small subset of cases exist that are attributable to rare inherited mutant genes. Since cancer is a multistep process, individuals with inherited mutations are at increased risk and tend to develop cancer at an earlier age than do those who acquire the mutations after birth. The majority of cancer cases appear to be due to sporadic mutations that may occur as a result of spontaneous genetic events, environmental events or interaction between genetic and environmental factors. Genetic epidemiologists will continue to work towards the discovery of inherited cancer genes as well as to define the interaction of genotype with the environment. It is this research that scientists hope will be used to help in the identification of individuals with an increased probability of developing cancer as well as in the development of prevention and treatment strategies for individuals found to be at risk.